1. Field of Invention
The present invention relates to a method and system for channel estimation, and specifically to a method and system for channel estimation involving signals from an ultra-long path.
2. Description of Related Arts
In an existing communication system, a sending end may send signals to a receiving end through different transmission paths (that is, channels). Therefore, the receiving end needs to perform ultra-long path detection to obtain accurate channel estimation. Currently, two methods exist for ultra-long path detection.
1. Judging whether an ultra-long path exists by observing a correlation sum for a long period of time using a correlation between cyclic prefixes and Orthogonal Frequency Division Multiplex (OFDM) symbols. Though having high reliability, this method can only determine an approximate position of the ultra-long path, and offers poor noise cancellation performance for channel estimation.
2. Performing blind estimation with a second-order matrix using all data and pilots in a frequency domain. This method has a disadvantage of high complexity. For more details, reference may be made to paper: Husen, S. A, Beggen S., Filippi A. et al, blind estimation of maximum delay spread in OFDM systems, vtc-2006 fall.
US Patent Application No. US20080198942 describes that when the channel length is greater than the number of pilots multiplied by the symbol rate, a window is placed to eliminate multi-path aliasing interference. The principle is described as follows.
A model based on discrete channels is:
      h    =                  ∑        i                                      ⁢                        a          i                ⁢                  δ          ⁡                      (                          i              -                              τ                i                                      )                                ,where ai and τi are a response and a delay corresponding to the ith transmission path respectively, and the corresponding frequency-domain response is:
            H      k        =                  ∑        i                                      ⁢                        a          i                ·                  exp          ⁡                      (                                          -                j                            ⁢                                                          ⁢              2              ⁢              π              ⁢                                                          ⁢              k              ⁢                                                          ⁢                                                τ                  i                                /                N                                      )                                ,where N is a Fast Fourier Transform (FFT) point number corresponding to the system. If a frequency label set of scattered pilots is Ssp={p|p=p0+lΔ}, where p0 is a position label of the first scattered pilot, Δ is an interval between neighboring pilot frequencies, and l is a serial number of a pilot, then a response at the pilot is:
      H    p    =                    ∑        i                                      ⁢                        a          i                ·                  exp          ⁡                      (                                          -                j                            ⁢                                                          ⁢              2              ⁢                                                          ⁢              π              ⁢                                                          ⁢              p              ⁢                                                          ⁢                                                τ                  i                                /                N                                      )                                =                  ∑        i                                      ⁢                        a          i                ·                  exp          ⁡                      (                                          -                j                            ⁢                                                          ⁢              2              ⁢                                                          ⁢                              π                ⁡                                  (                                                            p                      0                                        +                                          l                      ⁢                                                                                          ⁢                      Δ                                                        )                                            ⁢                                                τ                  i                                /                N                                      )                              
Without loss of generality, if p0=0,
                    H        p            ⁡              (        l        )              =                  H        p            =                                    ∑            i                                                          ⁢                                    a              i                        ·                          exp              ⁡                              (                                                      -                    j                                    ⁢                                                                          ⁢                  2                  ⁢                                                                          ⁢                  π                  ⁢                                                                          ⁢                  l                  ⁢                                                                          ⁢                  Δ                  ⁢                                                                          ⁢                                                            τ                      i                                        /                    N                                                  )                                                    =                              ∑            i                                                          ⁢                                    a              i                        ·                          exp              ⁡                              (                                                      -                    j                                    ⁢                                                                          ⁢                  2                  ⁢                                                                          ⁢                  π                  ⁢                                                                          ⁢                  l                  ⁢                                                                          ⁢                                                            τ                      i                                        /                                          (                                              N                        p                                            )                                                                      )                                                          ⁢        where, Np=N/Δ. It can be seen from the above equations that, if an ultra-long path satisfying τi=Np+τi′, τi′>0 exists, channel estimation using scattered pilots becomes fuzzy, as τi and τi′ in the above equations are corresponding to the same result. That is to say, in this mode of channel estimation using scattered pilots, the estimation range does not exceed Np. As shown in FIG. 1, once the multi-path signal received by the receiving end includes a signal from an ultra-long path, aliasing components may exist in the frequency-domain response of the channel during channel estimation using scattered pilots, resulting in fuzzy channel estimation. In this case, the method that performs channel estimation only according to scattered pilots fails.
Therefore, it is necessary to improve the existing method for channel estimation based on scattered pilots.